Method to shear a strip during the casting step

ABSTRACT

Method to shear a strip ( 15 ) during the casting step, wherein the casting speed is increased, with respect to a speed of a stationary regime, maintaining the thickness of said cast strip ( 15 ) constant to the value corresponding to the speed of a stationary regime, in order to determine the formation of a liquid core ( 16 ) which determines the re-melting of the adjacent skin and the breakage of the strip ( 15 ) due to the weight of the part of the strip ( 15   a ) located under the liquid core ( 16 ).

FIELD OF THE INVENTION

[0001] The invention concerns a method to shear a metal strip during theproduction thereof by means of continuous casting from the liquid metal.

[0002] The invention can be used in particular to remove a leading endsegment of a desired length of a strip emerging from a continuouscasting machine with two rollers, or, at the end of casting, toeliminate the terminal segment cast during the final transitory period.

[0003] The invention can also be used in any step whatsoever of thecasting, for example in emergency conditions to interrupt cleanly thestrip cast before discharging the liquid metal contained in the castingmachine, or to separate the production of two strips of differentthickness, or again when the machine is operating in stationaryconditions and it is desired to separate segments of strip forproduction reasons.

[0004] To be more exact, the invention concerns a shearing method bymeans of which it is possible to separate a desired segment of stripduring any step of the casting whatsoever, without using mechanicaldevices or other auxiliary assemblies provided specifically for suchoperation.

[0005] The invention is applied particularly but not exclusively incontinuous casting machines of the roller type (strip caster).

BACKGROUND OF THE INVENTION

[0006] It is known that the initial segment of strip, produced innon-stationary conditions, has an irregular conformation which couldmake it difficult for the strip to enter the pinch-roll or possiblerolling stands, which usually have a limited aperture.

[0007] This irregular conformation, moreover, has a quality which isunacceptable from the production point of view.

[0008] The final segment of the cast strip is also produced innon-stationary conditions and therefore has an unacceptable quality.

[0009] The solution to this problem is to remove a leading or trailingend segment, of the desired length, by means of shearing equipmentlocated immediately downstream of the continuous casting machine andupstream of the first rolling stand.

[0010] However, this solution, especially for steel, has technical andoperating problems which often make it impossible to use. In fact, theenvironment downstream of the continuous casting machine is verycritical, particularly because the strip, downstream of the castingrollers, is made to pass inside a closed environment with an inertatmosphere and kept at high temperature, in the case of steel around700÷1000° C., to prevent surface oxidation of the strip. This entailsconsiderable difficulties in installing and controlling shearingequipment such as shears or similar, and also in their functioning.

[0011] The need to interrupt a strip cleanly while it is being cast canoccur even in an intermediate step of the casting, for example inemergency situations, or to separate the production of strips ofdifferent thickness, or again, for production reasons, to divide thestrip into several segments during a casting in stationary conditions.

[0012] Documents U.S. Pat. Nos. 5,690,163 and 5,287,912 disclose amethod to shear a strip during the casting step by means of increasing,for a brief period, the distance between the casting rollers, keepingthe casting speed constant, said increase being then followed by arestoration of the distance adopted in the stationary regime. Thisincrease in thickness of the strip, maintaining the same casting speed,and hence maintaining the same time of contact between the strip and thecooled rollers, determines the formation of an inclusion of liquid metalbetween two thicknesses of solidified skin. This inclusion causes,downstream of the kissing point, the skins to melt again and the stripto break due to its own weight.

[0013] The present Applicant has devised and embodied this invention toovercome the shortcomings of the state of the art and to obtain otheradvantages as shown hereafter.

SUMMARY OF THE INVENTION

[0014] The invention is set forth and characterized in the main claim,while the dependent claims describe other innovative characteristics ofthe main embodiment.

[0015] The purpose of the invention is to achieve a method to shearstrip during the casting step which will allow to avoid using equipmentprovided specially for this purpose.

[0016] To be more exact, the invention proposes to obtain theseparation, in a substantially natural manner, of a segment of a desiredlength of strip, or the separation at a desired point of two segments ofstrip cast in stationary manner, exploiting the effect of the variationin the solidification of the liquid metal between the rollers, derivingfrom a variation in the casting speed and hence in the time of contactbetween the strip and the cooled surface of the rollers, keeping thethickness of the cast strip constant to the value corresponding to thespeed used in stationary regime.

[0017] Modifying the casting speed, with the same overall thickness ofthe cast strip as defined by the distance between the rollers,determines a variation in the conditions of heat exchange, and hence ofsolidification, of the strip due to the corresponding variation in thetime of contact between cooled surfaces of the rollers and liquid metal.

[0018] The thickness of solid skin which forms during the passage of theliquid metal on the cooled surfaces of the rollers is inverselyproportional to the casting speed raised to an exponent, which for steelis less than 1. An increase in the casting speed therefore determines areduction in the solidified thickness of the skins, just as a reductionin the casting speed determines an increase in the solidified thicknessof the skins, given the same cooling conditions of the rollers.

[0019] In the continuous casting of strip with two rollers, it ispreferable if solidification is complete at the point of minimumdistance between the rollers, the so-called “kissing point”.

[0020] With the method according to the invention, theend-of-solidification point is displaced below the kissing point,advantageously increasing for a brief interval of time the casting speedwith respect to a pre-set normal casting speed, and keeping the distancebetween the rollers constant to the value corresponding to this speed.After this time interval, in a first embodiment of the invention, thecasting speed is returned substantially to the starting value, possiblyafter a brief time of settling around said value.

[0021] According to a first variant, the increase in speed is precededby a deceleration step, so that this increased speed does not correspondto an excessive value of the rotation speed of the rollers and such asto entail too high an increase in the power of the motor which makesthem rotate.

[0022] This deceleration step is advantageously accompanied by acorresponding increase in the thickness of the cast strip, obtained bymodifying the distance between the casting rollers, in order to keep theconditions of heat exchange constant, and hence of solidification of theskins of the strip; this allows to prevent, in this case, too precociousa solidification of the strip caused by the reduction in the speed ofpassage between the rollers.

[0023] The portion of liquid metal, or inclusion, included between thekissing point and the end-of-solidification point which is obtained withthe brief increase in the casting speed, remains incorporated inside thestrip when the casting speed is returned to the original value.

[0024] This liquid portion affects a longitudinal segment of the stripwhere, downstream of the rollers, it causes the solidified skin to beheated, and takes it to a temperature near the re-melting point.

[0025] This longitudinal segment of strip therefore has very limitedmechanical properties, so that it is not able to support the weight ofthe part of the strip located underneath. This causes the breakage ofthe strip in correspondence with said segment and hence a segment of adesired length is naturally and cleanly removed.

[0026] The shearing position can be set at will by adjusting the momentat which the casting speed is increased.

[0027] The duration of the interval during which the casting isperformed at an increased speed is chosen so as to ensure the safety ofthe breakage of the strip; it depends on a plurality of parameters,including the thickness of the strip, the cooling parameters, the typeof material, the length and hence the weight of the segment to beremoved.

[0028] For steel, in the cases of most general use, an interval of about0-200 milliseconds at increased speed, associated with an accelerationtime comprised between about 50 and about 600 milliseconds, issufficient to create inside the strip a sufficient liquid zone whichwill allow the natural breakage of the strip due to the weight of itsleading end segment alone.

[0029] In another embodiment of the invention, after the increase inspeed which creates the inclusion of liquid metal downstream of thekissing point, the casting speed is kept at said increased value. Inthis case, once the inclusion has been formed with a constant thicknessof strip, the overall thickness of the strip is reduced, by reducing thedistance between the rollers, keeping the conditions of heat exchange,and hence of solidification of the skins of the strip, constant andsimilar to the stationary regime.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] These characteristics and advantages of the invention will becomeapparent from the following description of the preferential embodimentof the invention, given as a non-restrictive example with reference tothe attached drawings wherein:

[0031]FIG. 1 is a schematic view of a casting machine with rollerswherein the method according to the invention is used;

[0032]FIGS. 2a-2 d show graphs concerning the development of the castingspeed and the relative thickness of the strip in some possibleapplications;

[0033]FIG. 3 is a graph of the development over time of the internal andsurface temperature of a segment of strip, in correspondence of theinclusion, cast using the method according to the invention.

DETAILED DESCRIPTION OF SOME PREFERENTIAL EMBODIMENTS

[0034] With reference to the attached drawings, and particularly to FIG.1, number 10 denotes generally and schematically a continuous castingmachine with two rollers 11, wherein molten metal 12 is discharged bysuitable means (not shown here) and cast through a gap with anadjustable amplitude defined by the reciprocally facing surfaces of saidrollers 11.

[0035] Downstream of the rollers 11 there are usual, and not shown here,guide and extraction systems, normally with rollers, associated withcooling means.

[0036] The rollers 11, in known manner, are cooled at least on thesurface and the contact between the molten metal 12 and these cooledsurfaces causes the formation of two at least partly solidifiedhalf-skins 13 a, 13 b, which preferentially join in correspondence withthe kissing point 14, corresponding to the position of minimum distancebetween the two rollers 11.

[0037] At outlet from the rollers 11, thanks also to the cooling systemslocated downstream, a solidified strip 15 is obtained which is sent forrolling.

[0038] In order to remove a segment, for example a leading end segment15 a, of the strip 15, or a trailing end segment, or also to separatethe strip 15 at a desired intermediate point, the invention provides toincrease the casting and removal speed, starting from a time t₀, forexample from an initial nominal value v₁, which for steel can have avalue, for example, of about 40 m/min (as in the graph in FIG. 2a) to anominal value v₀ having a value for steel, for example, of about 52m/min, reached at time t₁. Time t₀ at which the increase in speed isstarted depends, for example, on the length of the leading end segment15 a, or of the trailing end segment, which has to be removed, or on theposition at which it is desired to separate two segments of strip 15.

[0039] The time needed to obtain said increase in speed, equal to t₁−t₀,for steel is normally comprised between 50 and 600 ms, for example about300 ms.

[0040] The increased speed is maintained until a time t₂, for aninterval (t₂ −t₁) advantageously very brief, comprised between about 0and, for steel, about 200 milliseconds. In the case shown in the graph,this interval is equal to 100 milliseconds.

[0041] Then this speed is reduced until, at time t₃, it again has thevalue v₁ of the stationary regime.

[0042] With this increase in speed from v₁ to v₀, and keeping thethickness of the strip constant and equal to s₁, for example in the caseof steel about 2.4 mm, the end-of-solidification point 114 is displacedbelow said kissing point 14, for a segment determined by the duration ofthe acceleration and deceleration ramps, by the value of maximum speedreached and by the time t₂−t₁ for which said increased speed ismaintained substantially constant.

[0043] This is because for the whole interval t₃−t₀, comprising theacceleration and deceleration ramps, the time of contact is reducedbetween the cooled surfaces of the rollers 11 and the correspondinghalf-skins 13 a, 13 b of the strip 15, with respect to the value whichwe would have at stationary speed, so that the solidified thickness ofsaid half-skins 13 a, 13 b is reduced correspondingly.

[0044] The portion of liquid metal 16 comprised between the kissingpoint 14 and the end-of-solidification point 114 which is obtained withthis brief increase in the casting speed remains incorporated inside thestrip 15 when the casting speed is returned to the original value.

[0045] The presence of the liquid inclusion 16 is visible in the lowergraph in FIG. 2a, wherein the line of dashes represents the developmentof the value of the overall solid thickness of the strip 15, obtained byadding together the thickness of the two half-skins 13 a, 13 b, whilethe continuous line represents the overall thickness of the strip 15.

[0046] This liquid portion 16 affects a longitudinal segment of thestrip 15 where, downstream of the rollers 11, it causes the solidifiedskin to be heated, taking it to a temperature near the re-melting point.

[0047] At this point please see the graph in FIG. 3, which shows how thesurface temperature of a segment of steel strip 15, cast incorrespondence with the inclusion, rises suddenly at outlet from therollers 11 due to the residual heat transmitted from the liquid core 16which is created due to the increase in the speed transmitted to thestrip 15. This surface temperature almost reaches the value of theinternal temperature and hence approaches melting temperature, causingthe solid skin formed before outlet from the rollers 11 to return to analmost liquid condition.

[0048] In this condition, there is a drastic reduction in the mechanicalproperties of the solid portion of the strip 15 around the liquid core16, so that the weight caused, in this case, by the leading end segment15 a causes the breakage of the strip 15 in correspondence with theliquid core 16 and hence said leading end segment 15 a is naturallyseparated.

[0049] When the increased casting speed interval is finished, the speedis reduced to return to the starting speed, in this case around 40m/min, in the case of steel.

[0050] According to the variant shown in FIG. 2b, the casting speed isfirst taken to a value v₁ less than that of the stationary regime v₀,and then increased again, for a brief period, to the value v₀ in orderto create the conditions which determine the formation of a liquidinclusion 16 as seen before. This deceleration step is performed so thatthe value of increased speed v₀ is not too high and does not lead to anexcessive increase in the power of the motors of the casting rollers.

[0051] In this case, since the thickness of the strip 15 whichsolidifies is inversely proportional to the casting speed raised to anexponent, which in the case of steel is less than 1, a correlatedincrease in the thickness of the cast strip 15 is associated with thereduction in speed, for example for steel from a value s₀ of about 1.8mm to a value s₁ of about 2.4 mm.

[0052] This increase in thickness, obtained by modifying the distancebetween the rollers 11, is necessary to maintain the cooling conditionsof the strip 15 substantially constant so that, until time t₀ when thespeed is increased, the conditions of solidification of the half-skins13 a, 13 b remain substantially corresponding to the stationary case.

[0053] In corresponding manner, if, after the inclusion 16 has beenformed, we want to return the casting speed to a higher value v₀,without this entailing the formation of further inclusions 16, it isnecessary, as can be seen in the right part of the graph in FIG. 2b, toreduce the thickness of the cast strip 15 from a value s₁ to a value s₀,also modifying, in this case, the gap between the rollers 11.

[0054] The necessary duration of the interval (t₃−t₀), comprising theramps and the time at increased speed, is chosen on each occasion toobtain the minimum conditions which ensure that a sufficient liquid core16 is formed so as to cause the breakage of the strip 15 due to theweight of the leading end segment 15 a, the trailing end segment or theintermediate segment underneath.

[0055] According to the embodiment shown in the graphs in FIGS. 2c and 2d, the shearing occurs simply due to the increase in the casting speed,from a value v₁ to a value v₀.

[0056] This increase determines the formation of an inclusion 16 ofliquid metal substantially in the same manner as seen before. However,after having formed said liquid inclusion 16, the casting speed is notreturned to the original value, but maintained at the increased value v₀(FIG. 2c). This maintained increased value is accompanied by acorresponding reduction in the thickness of the strip 15, from a values₁ to a value s₀, so that downstream of said inclusion 16 coolingconditions, and hence solidification conditions, are substantiallymaintained, corresponding substantially to the situation of a stationaryregime. This enables to obtain the shearing when the solid skin aroundthe inclusion 16 melts again and the weight of the segment of strip 15 acauses the strip 15 to break in correspondence with said inclusion 16.

[0057] In the application shown in FIG. 2d, in a manner substantiallyidentical to what is shown in FIG. 2b, the method provides a firstdeceleration step, accompanied by a correlated increase in the thicknessof the strip 15, in order to determine a start-up speed low enough tokeep the increased speed v₀ at a value which is not too high.

[0058] Deceleration is then followed by an acceleration at a constantthickness, which determines the formation of the inclusion 16 of liquidmetal. Then the speed is maintained constant at said increased value,but the thickness is reduced in correlated manner in order to maintainthe cooling and solidification conditions corresponding to the situationof a stationary regime.

[0059] As said above, it is obvious that this method can also be appliedsubstantially with the same principles and the same methods to shear thestrip 15 at any section other than the leading end segment, for exampleto remove a trailing end segment, or to crop the strip 15 at anyintermediate section.

[0060] Modifications and/or additions can be made to the method asdescribed heretofore without departing from the spirit and scopethereof.

1. Method to shear a strip (15) during the casting step as it emergesfrom a machine (10) for the continuous casting of liquid metalcomprising at least a pair of casting rollers (11), said machine (10)comprising at least cooling means able to achieve the solidification ofsaid strip (15), characterized in that it provides to increase thecasting speed with respect to a speed of a substantially stationaryregime, maintaining the value of speed increased for an interval(t₂−t₁), said increase in speed being performed keeping the thickness ofsaid cast strip (15) substantially constant to the value correspondingto the speed of a stationary regime, in order to determine a consequentformation of a liquid core (16) inside said strip (15) at least in itssegment passing at increased speed through said cooling means, saidliquid core (16) causing the at least partial re-melting of the skinadjacent thereto and the breakage of the strip (15) due to the weight ofthe part of the strip (15 a) located under said liquid core (16). 2.Method as in claim 1, characterized in that, when said interval atincreased speed is finished, the casting speed is returned to a valuesubstantially equal to the stationary regime speed.
 3. Method as inclaim 2, characterized in that, after returning the speed to a valuesubstantially equal to that of a stationary regime, and after obtainingthe shearing of the strip (15), the casting speed is again increasedand, in correlated manner, the thickness of the strip (15) is reduced inorder to obtain conditions of heat exchange and solidification of thestrip (15) substantially corresponding to those of a stationary regime.4. Method as in claim 1, characterized in that, having finished saidinterval at increased speed, the casting speed is maintained at saidincreased value, and a correlated reduction is performed in thethickness of the cast strip (15) in order to obtain conditions of heatexchange and solidification of the strip (15) substantiallycorresponding to those of a stationary regime.
 5. Method as in any claimhereinbefore, characterized in that, before said increase in speed, thecasting speed is temporarily reduced and that, in correlation with saidreduction in speed, the thickness of said strip (15) is increased inorder to obtain conditions of heat exchange and solidification of thestrip (15) substantially corresponding to those of a stationary regime,the function of said reduction in speed being to limit the powerrequired from the motors associated with said casting rollers (11). 6.Method as in any claim hereinbefore, characterized in that the durationof said interval (t₂−t₁) is at least a function of the thickness of thestrip (15) and of the type of metal cast.
 7. Method as in any claimhereinbefore, characterized in that the duration of said interval(t₂−t₁), in the case of steel, is comprised between about 0 and 200milliseconds.
 8. Method as in any claim hereinbefore, characterized inthat the time required to take the casting speed to an increased value,in the case of steel, is comprised between about 50 and about 600milliseconds.
 9. Method as in any claim hereinbefore, characterized inthat it is used to remove a leading end segment (15 a) of a strip (15)during the casting step.
 10. Method as in any claim from 1 to 8inclusive, characterized in that it is used to remove a trailing endsegment of the strip (15).
 11. Method as in any claim from 1 to 8inclusive, characterized in that it is used to crop the strip (15) at anintermediate section thereof, in emergency situations or to separate twostrips (15) of different thickness or to divide two segments of strip(15) for production reasons.